Twist capsules are devices that utilize flexible circuits wrapped around a shaft to transmit signals and power across a non-continuously rotating or oscillatory interface. These devices typically permit angular rotation over some limited range. Typical examples include twist capsules that are used to carry signals and power in gimbal assembles that exhibit oscillatory motion. Various twist capsules are shown and described in U.S. Pat. Nos. 4,693,527 A and 4,710,131 A. A high-frequency ribbon cable for use in a twist capsule is shown and described in U.S. Pat. No. 6,296,725 B1. The aggregate disclosures of each of these three patents are hereby incorporated by reference.
Twist capsules are noted for very long service lives, often in excess of 100-million full-excursion cycles of up to 360 degrees. Such long service lives require careful attention to the kinematics of the capsule.
Care should be exercised to maintain low stresses within the moving conductors, which are typically flex tapes in most twist capsules. Low stresses and long service lives in twist capsule service requires the use of highly-flexible conductors and dielectric materials. The physical characteristics that are necessary for promoting longevity of the twist capsules also place serious electrical constraints upon the types of signals that can successfully transmitted thereby, particularly with respect to high-speed data transmission. The primary electrical constraints are impedance-matching and high-frequency losses. Techniques have been developed to allow the transmission of moderately high speed digital data signals through these devices, primarily by the use of multilayer flexible circuits utilizing microstrip and stripline constructions, along with design strategies that optimize circuit impedance and control electromagnetic fields by utilizing ground plane structures. These techniques become less effective with increasing frequencies, and, with data rates above 1 Gbps, are especially problematic with transmission formats that require large bandwidths and relatively high transmission line impedances.
The use of thin conductors and dielectrics minimize flex tape thickness and enhance rotational life, but place severe constraints on the impedance and losses in the resulting transmission lines. The problems are especially acute with very high speed data transmission schemes, such as LVDS, Fibre Channel, XAUI, Infiniband, and others, that are designed around copper transmission lines with relatively high characteristic or differential impedances, with 100-Ohms being a very common value.
The current state of the art in long-life twist capsule design utilizes flex tape construction with thin polyimide dielectrics to achieve flexibility. Typical thickness values that promote long life also make it practically impossible to achieve impedance values on the order of 100-Ohms without creating extremely narrow traces. For example, a 100-Ohm differential impedance in a flex tape using 3-mil polyimide dielectric requires conductor trace widths of about 2-mils or less (i.e., about 0.002″ or about 0.05 mm). If this conductor width could be reliably manufactured, the circuit resistance would be extremely high, on the order of from about 5- to about 10-Ohms, or higher, for many typical twist capsules.
In addition, high-frequency losses become very important in high-speed data formats that require several gigahertz (“GHz”) of bandwidth, due to fast edge speeds that contain high-frequency harmonic energy. The very narrow conductors in high-impedance flex tapes have high losses at high frequencies, due to the skin effect that confines the high-frequency carriers to a thin skin on the conductors. In addition, traditional dielectric materials, such as polyimide, exhibit high losses at frequencies above 1 GHz, and also exhibit frequency-dependent dispersion, which causes different frequencies to travel at different speeds.
The net result of using a conventional flex tape transmission line construction at data transmission rates beyond about 1.0 Gbps, is severe attenuation of the high-frequency components and smearing of the digital data edge transitions due to dispersion. An eye pattern test of such a transmission can show a severely closed eye, or no eye at all. Each of these challenges to signal integrity of high-speed data signaling will be discussed below.
Typical flexible circuit construction utilizes etched copper traces sandwiched between layers of polyimide dielectric material. The dielectric losses that are a major constraint to high-frequency performance in flexible transmission lines are illustrated in FIG. 1. The parameter of interest is the loss tangent (ordinate), a convenient measure of high-frequency loss. As FIG. 1 shows, polyimide, which is the most popular dielectric material used in flex tape construction for twist capsules, is particularly lossy at high frequencies. Other dielectric materials, such as liquid-crystal polymer (“LCP”) and polytetrafluoroethylene (“PTFE”), have superior high-frequency properties, but are significantly more expensive and more difficult to manufacture. With the increased losses of high-frequency energy due to dielectric losses and skin effect, the edge speeds of high-speed data square waves can degrade to the point that data integrity may be compromised.
These dielectric materials do have the operational advantage of lower dielectric constants and lower dispersions, but high impedance transmission lines for data links of about 1.0 Gbps and beyond through flex tapes are still a very difficult challenge in the twist capsule environment. The mechanical design requirements of twist capsule and flex tape kinematics place practical constraints on the electrical design of flex tape transmission lines, and tend to favor lower impedance designs. Lower dielectric constant materials, such as PTFE and LCP, are advantageous for creating higher-impedance transmission lines, but the physical constraints required for long service life in a twist capsule are often at odds with the physical requirements of achieving high-impedance transmission lines structures, such as that required for 100-Ohm LVDS interfaces.
Accordingly, it would be generally desirable to provide an improved flex tape for use in a twist capsule that would allow the transmission of a higher bandwidth of signals.